In recent years, a storage battery system has been introduced into, for example, ordinary households, office buildings, and factories (e.g., Patent Literature 1). In the storage battery system, strings electrically connected with a common DC bus in parallel are provided. Each of the strings is charged with surplus power supplied from, for example, a solar battery, via the DC bus. The power charged in the strings is supplied via the DC bus to, for example, electrical equipment installed in ordinary households, office buildings, factories, and others.
The strings each include a DC-DC converter and battery packs (storage batteries) which are electrically connected in series. The DC-DC converter includes a voltage sensor for detecting a voltage value at the connection point between the DC-DC converter and the DC bus. The DC-DC converter controls the amount of charge in and the amount of discharge from each of the battery packs, based on the voltage value detected by the voltage sensor.